Is "going back to school" a thing in Germany? From what I know about that education system I'm guessing not, but in the US that's a standard answer.
"I want to move to be closer to my aging parents." - might work. If you haven't shared much about your life, it's an easy lie. And it's dull and depressing, so the odds of followup questions are slim. I hear it a lot when interviewing job candidates in our area.